According to the state of the art, an additive, affecting the degree of abhesiveness, is referred to as a controlled release agent or a controlled release additive. The corresponding abbreviation is CRA. For convenience, the abbreviation CRA is used in the specification.
Abhesive coating compositions are used on a large scale for coating particularly 2-dimensional materials, in order to decrease the tendency of products to adhere to these surfaces. Abhesive coating compositions are used, for example, to coat paper, films or sheets, which are to act as backing for pressure-sensitive labels. The labels, provided with a pressure-sensitive adhesive, adhere to the coated surface to a still sufficient extent to enable the backing sheets with the pressure sensitive labels to be handled. The adhesion of the pressure-sensitive labels to the backing sheets must be high enough, so that, when the labels are applied by machine on, for example, packing drums, there is no premature detachment as the backing sheet with the labels is passed over guide rollers. On the other hand, however, it must be possible to pull the labels from the coated backing sheet without significantly affecting their adhesive power for later use. Further possible applications for abhesive coating compositions are packing papers, which are to be used, particularly, for packing sticky goods. Such abhesive papers, sheets or films are used, for example, to pack foods or industrial products such as bitumen.
A further use for abhesive coating compositions exists in the manufacture of tapes, for example, for so-called disposable diapers. If the abhesiveness is too high, that is, the release value too low, the diaper does not remain closed reliably. If the abhesiveness is too low and, with that, the release value too high, the fastening cannot be undone without tearing the diaper.
An important requirement of abhesive coating compositions therefore is the possibility of being able to adjust the degree of abhesiveness and, with that, reciprocally, the force required for the separation in accordance with the intended application.
The literature, in which the composition, manufacture and use of abhesive coatings are described, is very extensive. The following patents and Offenlegungsschriften are named as being representative of the state of the art.
The German patent 29 48 708 discloses a method for the preparation of organopolysiloxanes, modified with pentaerythritol triacrylate or pentaerythritol trimethacrylate esters and synthesized from organochloropolysiloxanes, optionally with the addition of HCl-binding utilizing agents, with the distinguishing feature that organopolysiloxanes having the formula ##STR1## (R.sup.1 =alkyl with 1 to 4 carbon atoms, vinyl and/or phenyl, with the proviso that at least 90 mole percent of the R.sup.1 groups are methyl; a has a value of 1.8 to 2.2 and b a value of 0.004 to 0.5) are reacted first with, based on the SiCl groups, at least 2-molar amounts of a dialkylamine, the alkyl groups of which in each case have 3 to 5 carbon atoms, the carbon atoms adjacent to the nitrogen carrying not more than 1 hydrogen atom, and the reaction product is reacted with at least equimolar amounts of pentaerythritol triacrylate or pentaerythritol trimethacrylate and the reaction product is then separated by known methods from solid components suspended in the product.
The abhesive properties of the acrylate ester-modified organopolysiloxanes are improved by removing the suspended solid components from the reaction product. In the aforementioned patent 29 48 708, organopolysiloxanes with SiH groups are named as modifiers, particularly for lowering the viscosity of the coating compositions. Free radical starters, such as benzophenone, its oxime or benzoin ether are added in an amount of 2 to 5% by weight, based on the modified siloxane, to the coating compositions, which are then cured under a suitable source of UV radiation.
It is an object of the European publication 0 168 713 to increase the adhesion of the resins to the substrate and, at the same time, to improve the abhesive surface properties. According to this European patent, this combination of properties is found in organopolysiloxane mixtures modified with (meth)acrylate esters, which are characterized in that they consist essentially of an equilibrated organopolysiloxane with, on the average, more than 25 and fewer than 200 silicon atoms and, in addition, contain 2 to 30% by weight of organopolysiloxane with, on the average, 2 to 25 silicon atoms and 2 to 30% by weight of organopolysiloxane with, on the average, 200 to 2,000 silicon atoms. These teachings are based on the knowledge that, if the content of low molecular weight components is too small, there is inadequate adhesion to the substrate. If, however, higher molecular weight components are lacking in the mixture, the abhesiveness of the mixture is inadequate. If the content of low molecular weight and high molecular weight components is too large, the mixture does not cure under the given conditions. It is therefore important to adhere exactly to the conditions for selecting the three components of the organopolysiloxane mixture.
Adhering to the requirements of the aforementioned three-component system proved to be difficult. An attempt was therefore made to achieve the same result with a mixture consisting of two siloxane components, which can be synthesized with better reproducibility. This led to the German patent 38 41 843, which relates to a mixture of organopolysiloxanes, which are modified with (meth)acrylate esters and characterized in that they contain
60 to 95% by weight of a modified organopolysiloxane A with, on the average, 5 to 50 silicon atoms, and PA1 5 to 40% by weight of a modified organopolysiloxane B with, on the average, at least 50 silicon atoms, PA1 1. Satisfactory adhesion to the backing that is to be coated; PA1 2. High-curing rate on the backing; PA1 3. Chemical and physical resistance of the cured coating; PA1 4. High flexibility of the cured coating; PA1 5. Abhesive properties with respect to adhesive products, adaptability of the abhesive coating to the chemical character of the adhesive; and PA1 R.sup.2 is the same as R.sup.1 or represents the R.sup.3 group, the R.sup.3 group consisting of PA1 with the proviso that at least 1.5 epoxy groups are contained in the average molecule, PA1 a has a value of 1 to 1000, and PA1 b has a value of 0 to 10, PA1 the R.sup.2 groups can partly have the meaning of the R.sup.1 groups, the remaining R.sup.2 groups consisting to the extent of 70 to 100% of hydroxy-functional groups of the formula EQU --CH.sub.2 (CR.sup.3.sub.2).sub..xi. --(OCH.sub.2 CHR.sup.4).sub..nu. --OH, EQU --CH.dbd.CH--CR.sup.3.sub.2 --OH PA1 and/or groups of the formula ##STR5## wherein PA1 with the proviso that at least 1.8 hydroxy-functional R.sup.2 groups are contained in the average molecule, PA1 a has a value of 1 to 1000, and PA1 b has a value of 0 to 10, PA1 1. The abhesiveness of the organopolysiloxanes, which are to be used pursuant to the invention, increases after their curing with the number of (meth)acrylate ester groups in the polymer molecule. As the cross-linking density increases, the glass transition temperature of the cured coating increases and the flexibility of the coating decreases. At the same time, the chemical and physical stability of the cured coating increases. PA1 2. As the proportion of monocarboxylate ester groups, which are free of double bonds capable of polymerizing, increases, the abhesiveness decreases and the adhesion to the backing improves. This decrease in abhesiveness is additionally reinforced by hydroxyl groups of unreacted, hydroxy-functional R.sup.2 groups, which may still be present. By these means, the abhesive coating can also be adapted to the chemical character of the adhesive. PA1 R.sup.2 partially can be the same as the R.sup.1 groups, the remaining R.sup.2 groups being PA1 with the proviso that at least 1.8 epoxy groups are contained in the average molecule PA1 a has a value of 1 to 1000 and PA1 b has a value of 0 to 10, PA1 (A) 100 parts by weight of an epoxy-functional polyorganosiloxane, which comprises units of the formula R.sup.1 R.sup.2 SiO, wherein R.sup.1 is a hydrogen group or a univalent hydrocarbon group and R.sup.2 is a hydrogen group, a univalent hydrocarbon group or a univalent epoxy-functional organic group, with the proviso that at least two of all the organic groups are such univalent, epoxy-functional organic groups, PA1 (B) 1 to 80 parts by weight of a copolymer, which regulates the peel force, is soluble in component (A) and is selected from a group, which consists of PA1 wherein PA1 wherein PA1 (C) a catalytically effective amount of an onium salt as photoinitiator. PA1 (A) an epoxy-functional diorganopolysiloxane of the general formula EQU R.sub.2 R.sup.1 SiO(RR.sup.1 SiO).sub.x (R.sub.2 SiO).sub.y SiR.sub.2 R.sup.1 PA1 wherein PA1 (B) a catalytic amount of a photocatalyst or a combination of photocatalysts, and PA1 (C) about 1 to about 30% by weight, based on (A), of a siloxane resin which contains univalent R.sup.2.sub.3 SiO.sub.1/2 - and R.sup.3 R.sup.2 SiO.sub.1/2 -units and quadrivalent SiO.sub.4/2 units, the ratio of the univalent to the quadrivalent units being 0.6:1 to 1.1:1, and wherein R.sup.2 in each case is a univalent hydrocarbon group with not more than 2 carbon atoms, R.sup.3 is a univalent epoxy-functional organic group with about 2 to 20 carbon atoms and wherein the R.sup.3 R.sup.2.sub.2 SiO.sub.1/2 units are present in an amount of 1 to 90%, based on the total number of univalent units present. PA1 a) The abhesiveness should be adjusted so that it is almost independent of the rate at which the adhesive is pulled from the release coating. PA1 b) A uniform release behavior should be attained with no or only a slight development of noise; this means that oscillating release fluctuations, which result in a crackling noise (so-called "zip" or also "slip stick"), are avoided during the peeling process. PA1 R.sup.2 is an alkyl group with 1 to 4 carbon atoms or an alkenyl group with 2 to 6 carbon atoms, PA1 R.sup.3 is the R.sup.1 group or an alkoxy group with 1 to 4 carbon atoms or a hydroxy group, PA1 R.sup.7 is a methyl or hydrogen group, PA1 b has a value of 0 to 50, PA1 the quotient x/(y+z) is equal to 0.5/1.0 to 1.5/1.0 and PA1 the quotient z/x is equal to 0/1.0 to 0.4/1.0. PA1 [(CH.sub.3).sub.3 SiO.sub.1/2 ].sub.12 [SiO.sub.2 ].sub.20 [O.sub.3/2 SiOH].sub.2 PA1 [(CH.sub.3).sub.3 SiO.sub.1/2 ].sub.25 [SiO.sub.2 ].sub.25 [O.sub.3/2 Si--O--CH.sub.3 ].sub.1 PA1 [(H.sub.2 C.dbd.CH) (CH.sub.3).sub.2 SiO.sub.1/2 ].sub.38 [SiO.sub.2 ].sub.33 PA1 [(CH.sub.3).sub.3 SiO.sub.1/2 ].sub.5 [SiO.sub.2 ].sub.4 ]O.sub.3/2 Si--CH.sub.3 ]. PA1 R.sup.5 has the meaning of the R.sup.4 group or represents the ##STR13## group, in which R.sup.6 is a divalent, optionally unsaturated hydrocarbon group with up to 20 carbon atoms, which can be interrupted by an oxygen atom, and PA1 b has a value of 0 to 50. PA1 (1) Linear or branched aliphatic hydrocarbon groups with 1 to 12 carbon atoms, PA1 (2) Linear or branched olefinically unsaturated hydrocarbon group with 2 to 20 carbon atoms, PA1 (3) Cyclic, aliphatic hydrocarbon group with 6 to 20 carbon atoms, PA1 (4) Aryl or alkaryl groups with 6 to 20 carbons atoms.
with the proviso that the number of silicon atoms in the average molecule of the modified organopolysiloxane B is at least twice the number of silicon atoms in the average molecule of the modified organopolysiloxane A.
In the course of the development of further radiation-curable coating compositions based on organopolysiloxanes with (meth)acrylate ester groups, further requirements, which the properties of such compounds must meet, were observed. From the European publication 0 281 681, it can be inferred that the following combination of properties is aimed for:
6. Adjustability of the desired degree of abhesiveness.
The object of this European patent 0 281 681 are polysiloxanes, which have (meth)acrylate ester groups linked over SiC groups and are obtained by the reaction of epoxy-functional polysiloxanes of the general formula ##STR2## wherein R.sup.1 are the same or different and represent low molecular weight alkyl groups with 1 to 4 carbon atoms or phenyl groups,
70 to 100% of epoxy-functional groups, and PA2 30 to 0% of alkyl groups with 2 to 10 carbons atoms or hydrogen groups, PA2 R.sup.3 are the same or different and in each case represent a hydrogen or alkyl group with 1 to 4 carbon atoms, PA2 R.sup.4 are the same or different and in each case represent a hydrogen or alkyl group with 1 to 10 carbon atoms, and the subscripts PA2 n=0 to 10 and PA2 m=0 to 40 and PA2 30 to 0% of optionally substituted alkyl groups with 2 to 20 carbon atoms and/or hydrogen groups, PA2 up to 70 to 100% conventional epoxy-functional groups and PA2 up to 30 to 0% alkyl groups with 2 to 10 carbon atoms or hydrogen groups, PA2 (i) a copolymer corresponding to a structure unit MQ or M.sup.A Q, PA2 (ii) a copolymer corresponding to a structure unit M.sup.A T, MT.sup.A or M.sup.A T.sup.A, and PA2 (iii) a copolymer corresponding to a structure unit M.sup.A DQ, MD.sup.A Q or M.sup.A D.sup.A Q, PA2 M is an R.sup.3.sub.3 SiO.sub.1/2 unit, PA2 M.sup.A is an R.sup.3.sub.2 R.sup.4 SiO.sub.1/2 unit, PA2 D is an R.sup.3.sub.2 SiO unit, PA2 D.sup.A is an R.sup.3 R.sup.4 SiO unit, PA2 T is an R.sup.3 SiO.sub.3/2 unit, PA2 T.sup.A is an R.sup.4 SiO.sub.3/2 unit, PA2 Q is an SiO.sub.2 unit, PA2 R.sup.3 is a univalent hydrocarbon group free of olefinic unsaturation, and PA2 R.sup.4 is an alkenyl group, and PA2 R in each case is a lower molecular weight alkyl group with 1 to 8 carbon atoms, PA2 R.sup.1 in each case is a univalent, cycloaliphatic, epoxy-functional organic group with 2 to about 20 carbon atoms, PA2 x is a number from about 1 to about 50, and PA2 y is a number from about 1 to about 1000, PA2 R.sup.7 is a methyl or hydrogen group,
with such amounts of a mixture of acids, consisting of 10 to 90 mole percent of (meth)acrylic anhydride and 90 to 10 mole percent of (meth)acrylic acid, the sum having to add up to 100 mole percent, that there are 0.8n to 1.9n and preferably 1.1n to 1.9n acid equivalents for n epoxide equivalents, at elevated temperatures, optionally in the presence of solvents and conventional esterification catalysts.
In the reaction of epoxy-functional, modified siloxanes with an acid mixture of (meth)acrylic anhydride and (meth)acrylic acid, (meth)acrylate esters are formed according to the following equations: ##STR3##
Depending on the composition of the mixture consisting of (meth)acrylic anhydride and (meth)acrylic acid and the amount of mixture used (based on the epoxide group), the organopolysiloxanes, modified pursuant to the invention, have predetermined amounts of (meth)acrylate diester groups, (meth)acrylate monoester groups, hydroxyl groups and, optionally, unreacted epoxide groups. Since it has been observed that the abhesiveness increases with the number of (meth)acrylate ester groups while the compatibility with the substrate improves with the content of hydroxyl groups and, optionally, epoxide groups, these mutually opposite properties can easily be brought into a balanced relationship desired for the application.
Furthermore, the cross-linking density of the cured siloxanes can be influenced by the ratio of monoester groups to diester groups. Differences in cross-linking density result in different physical properties; for example, if the cross-linking density is increased, the hardness and the mechanical resistance are increased and, if the cross-linking density is decreased, the glass transition temperature and elasticization are lowered.
A further possibility for influencing the abhesive properties is given by the fact that up to 30% of the R.sup.3 groups can optionally be substituted alkyl groups with 2 to 20 carbon atoms. Due to the presence of alkyl groups with 2 to 20 carbon atoms, the organic character of the modified polysiloxanes is increased. In this connection, it can generally be assumed that the abhesive properties of the polysiloxane are selectively reduced by increasing the content of alkyl groups as well as by increasing the chain length of the alkyl groups.
A further possibility for influencing the abhesive properties of such coating compositions can be taken from the German 38 10 140. The object of this patent is the use of polysiloxanes with (meth)acrylate ester group linked over SiC groups, obtainable by the reaction of polysiloxanes of the general, average formula ##STR4## wherein the R.sup.1 groups are the same or different and in each case represent low molecular weight alkyl groups with 1 to 4 carbon atoms or phenyl groups,
with, based on the hydroxyl groups, 0.4 to 0.9 molar amounts of (meth)acrylic acid and up to 0.6 molar amounts of a monocarboxylic acid, which is free of double bonds capable of polymerizing, wherein the sum of the molar amounts of acids must not exceed 1.0, under conventional esterification conditions, as radiation curable coating components.
For the organopolysiloxanes, which are to be used pursuant to this patent, 40 to 90 mole percent of the hydroxy-functional R.sup.2 groups are present in the form of their (meth)acrylate esters. Up to 60 mole percent of the hydroxy-functional R.sup.2 groups can be present in the form of monocarboxylate esters free of double bonds capable of polymerizing. Moreover, depending on the proportion of monocarboxylic acid used for the esterification, the hydroxy-functional R.sup.2 groups can moreover be present unchanged. The ratio of the (meth)acrylate ester groups and monocarboxylic ester groups, derived from the R.sup.2 groups to the unchanged hydroxy-functional R.sup.2 groups arises out of the nature and amount of the (meth)acrylic acid/monocarboxylic acid mixture used for the esterification.
By these means, a person well versed in the state of the art is able to adjust, in the desired manner, the properties of the organopolysiloxanes, which are to be used pursuant to the patent:
In the German patent 38 20 294, as in the German patent 38 10 140, polysiloxanes with (meth)acrylate ester groups linked over SiC groups are described, which can be obtained by the reaction of polysiloxanes of the general, average formula ##STR6## wherein R.sup.1 are the same or different and in each case represent lower molecular weight alkyl groups with 1 to 4 carbon atoms or phenyl groups,
with, based on the epoxide groups, 0.4 to 0.9 molar amounts of (meth)acrylic acid and the reaction of the remaining epoxide groups with a monocarboxylic acid with 2 to 12 carbon atoms, which is free of double bonds capable of polymerizing.
For the state of the art named so far, it is necessary to synthesize a new siloxane resin, modified so as to be adapted approximately to the intended use, for each special, intended use. This leads to a large number of products, the manufacture of which, relative to the amount used, is comparatively expensive.
It is therefore more desirable to produce a basic resin or a corresponding mixture of basic resins of high abhesiveness (which is the reciprocal of a low release force) and to adjust the basic resin or the mixture of basic resins with the help of an additive, namely a controlled-release agent (=CRA) selectively to the desired lesser abhesiveness, so that the abhesiveness decreases in proportion to the concentration of CRA. The basic resin of very high abhesiveness can thus theoretically be adjusted to any abhesiveness with the CRA.
Such CRA products are already known and described, for example, in the European patent applications 0 464 706 and 0 473 995.
The object of the European publication 0 464 706 is a UV-curable silicone preparation consisting of
The copolymer (B) is usually also referred to as a so-called MQ or MQT resin. In addition, this resin fulfills the role of a CRA.
However, if such MQ resins are used in an analogous manner as CRA for radiation-curable, abhesive coating compositions based on (meth)acrylate-modified polysiloxanes, satisfactory results are not obtained, since the abhesiveness of the radiation-cured preparation changes upon storage (ages); in particular, the abhesiveness decreases and the release force increases. This effect is all the more pronounced if the radiation-cured preparation is stored as a laminate, that is, in a composite with the pressure-sensitive adhesive. In particular, after storage for a longer time or at an elevated temperature, this can lead to an interlocking of the material, so that the release coating and the pressure-sensitive adhesive can no longer be separated from one another without damaging the product. Such an abhesive release coating therefore does not have an adequate shelf life.
The European patent application 0 473 995 relates to a UV-curable epoxy-functional silicone preparation with controlled-release behavior, which contains the following components:
Here also, the CRA is a so-called MQ resin. The addition of such epoxy-functional MQ resins to (meth)acrylate-modified polysiloxanes also leads to coating compositions, which have the disadvantages of an inadequate shelf life, which have already been mentioned above. Moreover, such epoxy-functional MQ resins are synthesized by means of a multistep method involving the hydrosilylation of previously synthesized SiH-functional MQ resins with vinyl cyclohexene oxide. Admittedly, these reactions can be carried out; however, they are not very practicable from a reaction technology point of view. Because of the high molecular weight structure of the MQ resins that are to be modified and the fact that, for reasons of compatibility, solvents are employed, the reaction proceeds very slowly for kinetic reasons and leads to incomplete conversions when the usual excess of educts and concentrations of catalysts are used.
The present invention is therefore concerned with the technical problem of finding controlled-release agents (CRA), which do not have these disadvantageous properties of the change in the abhesiveness upon storage or, at the very least, have these properties to a significantly lesser extent.
The invention is furthermore concerned with the technical problem of synthesizing these controlled-release agents as reproducibly as possible, with the simplest possible method, in high yield and using easily obtainable starting materials. Such CRAs should meet the following technically important requirements:
Surprisingly, it has now been found that the disadvantages listed above can be avoided by the addition of special CRAs to (meth)acrylate-modified polysiloxanes, such CRAs, synthesized pursuant to the invention, also fulfilling the economic and technical requirements.